1. Field of the Invention
The present invention relates in general to the measurement of the rate of decay, or capture, of thermal neutrons in earth formations and, more particularly, to new and improved methods and apparatus for measuring thermal neutron decay time constants and related capture cross sections of earth formations traversed by a well bore.
2. The Prior Art
Heretofore, pulsed-neutron capture logs have provided measurements of thermal neutron capture characteristics of earth formations, e.g. the thermal neutron decay time constant (.tau.) and its correlative the macroscopic capture cross section (.SIGMA.), which have proven useful in differentiating between oil or gas-bearing formations and water-bearing formations. Such logs are especially useful in recognizing the presence of hydrocarbons in cased formations, and to detect changes in water saturation during the production life of a well.
Thermal neutron characteristic measurements are typically made by irradiating a formation with bursts of fast (e.g. 14 Mev) neutrons and following the decay of the thermal neutron concentration in the formation by counting the gamma rays emitted by formation nuclei upon the capture of thermal neutrons during discrete time intervals, or gates, following each neutron burst. In one prior tool disclosed in U.S. Pat. No. 3,379,882 to A. H. Youmans, the capture gamma rays are measured during two gates which are fixed both in time of occurrence after the burst and in duration. Although affording useful information in formations of average decay times, the Youmans fixed-gate system tends to yield unreliable measurements where the decay time of the formation is either very long or very short. Moreover, the gamma ray count rate measurement during the second fixed-gate is sometimes subject to excessive statistical variation, particularly in short decay time formations. In an important advance over the fixed-gate system, W. B. Nelligan in U.S. Pat. No. 3,566,116 (now U.S. Pat. No. Re. 28,477) patented a sliding-gate system in which three measurement gates are utilized and in which the time-after-burst occurrence and duration of all of the gates are automatically varied, in a feed-back loop operation, according to the currently measured value of the decay time constant. The first two gates are timed to detect capture gamma rays from the formation and the third gate is timed to detect background gamma rays. This system operates properly to position the gates for optimum background-corrected measurements over a wide range of decay times .tau. and cross sections .SIGMA., thereby avoiding the deficiencies in respect of unreliability and statistical variation encountered in the fixed-gate system in cases of extreme decay rates. For still better results, Nelligan further provides that the duration and repetition rate of the neutron bursts could also be varied as a function of the currently measured decay time value. This affords the added advantage of maximizing the duty cycle of the neutron generator in a manner consistent with accurate measurement of the decay time value of the formation being logged. Later embodiments of the Nelligan sliding-gate concept are described in U.S. Pat. No. 3,662,179, granted May 9, 1972 to Frentrop et al., and U.S. Pat. No. 3,890,501 granted June 17, 1975 to C. W. Johnstone. Thermal neutron decay time logging, in accordance with the Nelligan sliding-gate technique as described in the aforementioned patents, is provided commercially by Schlumberger Well Services, and has become a widely accepted and important cased-hole service.
It is desirable, however, to improve still further this service. Specifically, it is desirable to provide still greater statistical precision in the measurements of .tau., .SIGMA. and background by improvement in the manner of detection of the rate of decay of the thermal neutron concentration. Also, the infinitely variable, feed-back loop type of operation previously used with the Nelligan sliding-gate system is sometimes subject to "jitter" when low counting rates are encountered. That is to say, variations in the settings of the measurement gates and the neutron bursts sometimes result from statistical variations in the gamma ray count rates rather than as the result of any change in the decay time of the formation under investigation. Again, where the decay time drops sharply, such as at bed boundaries, the feed-back loop of the sliding-gate tool sometimes, though infrequently, fails to change the timing of the gates fast enough to keep up with the fall off in the gamma ray count rate. This could result in the tool measuring insufficient count rates for the feed-back loop to work properly, which situation could in turn leave the gates and bursts "latched" at positions later after the burst than would be optimum for the new decay time. Although this situation can be readily overridden manually and the gates quickly restored to the proper positions, it is desirable to avoid such inadvertent "latching" of the .tau. computation circuits. It additionally is desirable to provide for the measurement of all decay time values over the full .tau. range normally encountered, e.g. from &lt;50 .mu.sec to &gt;600 .mu.sec, without any discontinuities.